In an IT-system (French Isolé Terre) or an IT-network, conductive housings of the devices are earthed, but not the star point of a feeding transformer. Thus (with small networks), no dangerous touch voltage occurs given an insulation fault between a conductor of the network and the housing or earth. An insulation monitoring must detect this first fault, before a second, potentially dangerous fault occurs.
With IT-networks with electrical (clocked) drives, fault currents arise due to the relocation of charge carriers in the cable and motor capacitances, and these fault currents behave similarly to fault currents that are due to insulation defects. Conventional insulation testing apparatus are interfered with on account of this, which leads to misleading error messages with regard to the condition of the insulation. The setting of the warning limits and the fault limits is rendered more difficult and partly rendered impossible, due to the dependency of these capacitive fault currents on the number of drives, their momentary operating conditions, and on the manner in which the cables are laid. Making this more difficult is also the fact that these capacitive fault currents may change on account of the aging of the insulation, dirt and humidity, so that warning limits and fault limits which were once set in an optimal manner, must also be reset later, in order to avoid misleading error messages.
Insulation monitoring apparatus must themselves periodically monitor their own manner of functioning. As a rule, additional circuit parts are incorporated for this, which in the case of a fault, may themselves lead to a loss of insulation, which increases the failure rate of the complete system.
Insulation defects may occur everywhere in the system, and are therefore difficult to localise. In complex installations, this leads to very high costs for fault correction, e.g. in that the wrong components are repeatedly exchanged, since the exact defect cause may not be determined in the field. Likewise, during maintenance work, on account of a sporadically occurring increase and reduction of the capacitive fault currents, insulation damage which is indeed present may not be recognised, or only too late, since the fault diagnosis may only be carried out in a very imprecise manner.
An arrangement is presented in DE 42 03 299 A1, which uses a part of the measurement circuit present in the electrical drives (inverter devices) for the measurement of the DC voltage in the intermediate circuit, for monitoring the insulation. There, this method is applied for applications in housings encapsulated in a pressure-tight manner, wherein the monitoring limit values are achieved by setting resistance values, and the suppression of the capacitive fault currents produced by the switching of the drives is achieved by low-pass filtering. This method has the disadvantages mentioned in the introduction, and cannot determine the location of the insulation error.
DE 43 39 946 A1 and DE 101 06 200 C1 show insulation monitoring by way of injecting a measurement voltage via an ohmic measurement circuit. The measurement voltage is produced by a special generating circuit.
DE 696 24 409 T2 shows insulation monitoring with electrical vehicles, with which a measurement alternating voltage is fed into the network capacitively and the current which is fed in this manner is measured.
DE 195 03 749 C1 describes a measurement bridge for determining an asymmetry in the on-board network of an electric vehicle, which detects a shifting of the positive or the negative feed voltage.
EP 0 490 388 discloses a fault current detector for an inverter, which by way of shunt resistors, measures the current through each of the half-bridges, and signalizes an earth fault in the case that an excess current occurs, and simultaneously all switches of the upper half-bridge or all switches of the lower half-bridge are closed. This measurement of insulation defects by way of current sensors may only be used for the hard earth fault in a TN-network (Terre Neutre network). However, given an insulation defect, only very small fault currents determined by the measurement circuit occur in an IT-network, so that these fault currents are orders of magnitude smaller than the currents occurring in normal operation, so that this idea is of no relevance with regard to IT-networks.
EP 1 336 528 A2 shows a detection of ground errors by coupling out the voltage of a branch of an intermediate circuit (or DC link) by means of a capacitance and a voltage divider, rectifying the voltage, filtering it and inputting it to a comparator. In other variants, the branch voltage is captured over a measurement transformer, or the current through a grounding capacitor (C3) is measured. In each case it thus only possible to detect the presence of an alternating voltage, as it may occur due to a ground error on a branch. Thus, there is no measurement of the branch voltage between the branches, and there further only is a filtered measurement, that is, a measurement that does not provide an actual, momentary value of the branch voltage. The operating condition of the inverter is not considered.
U.S. Pat. No. 5,686,839 shows a method for detecting ground errors which is executed when the motor is not powered. Dedicated measurement resistors (that is, they are not required for normal operation of the converter) are arranged between the electrical ground and the intermediate circuit branches. A ground fault causes the potential of the intermediate circuit to be defined with respect to ground, and a ground fault current flows through at least one of the measurement resistors, so that the voltage measured across these resistors indicates a ground fault. In order to drive this current, for example, all the upper or all the lower switches of the inverter are closed. In another variant, all six switches are opened and closed individually. The difference of measurement voltages can be used for the fault detection, but it is only possible to detect ohmic ground faults. Detecting faults during operation of the inverter is considered to be disadvantageous.
JP 2000 116144 aims to detect a ground fault during operation of an inverter, and is based on the measurement of the current through the intermediate circuit. Furthermore, circuitry is required which detects a particular state of the inverter, in which all the switches of a half-bridge of the inverter are closed, i.e. in a conducting state.